Earthquakes at mid-ocean ridges reflect the active magmatic and tectonic processes that form
new oceanic crust. Studies of large earthquakes observed on land and smaller earthquakes
observed locally or regionally by ocean bottom seismometers or autonomous underwater
hydrophones have greatly contributed to our understanding of the structure and active spreading
processes at the mid-ocean ridges of the Atlantic and Pacific Ocean opening with velocities
in excess of 25 mm yr–1. At spreading rates below 20 mm yr–1 full rate, the appearance
and the accretion processes of mid-ocean ridges undergo fundamental changes as the melt
supply is drastically reduced. The active spreading processes at these so-called ultraslow
spreading ridges are still poorly known, as the main representatives, the Arctic Ridge System
and the Southwest Indian Ridge, are poorly accessible and neither autonomous underwater
hydrophone nor ocean bottom seismometer records of local seismicity are available. In an
attempt to compare on a large scale the accretion style of ultraslow spreading ridge sections, I
analyse the teleseismically recorded seismicity in 11 sections of the Arctic Ridge System and
the Southwest Indian Ridge spanning altogether 7200 km. Epicentres located within 30–35 km
of the rift axis were extracted from the catalogue of the International Seismological Centre for
a time period of 35 yr. On the basis of a single-link cluster analysis, I identified 27 swarms
with eight or more events. These swarms occur almost exclusively at centres of focussed
magmatism suggesting that the swarms are probably initiated by magmatism. Normal faults
along several tens of kilometres surrounding the volcanic centres react in large earthquakes
(M > 5) to dyke emplacement. The routine generation of large earthquakes in the cold, brittle
lithosphere of ultraslow spreading ridges makes the teleseismic record a valuable means to
study ultraslow accretion processes and to provide a global framework for the interpretation
of the limited local and regional seismicity studies.